To some, Asif Ali Khan Zardari is a corrupt, bullying chancer who was a political liability for his late wife, Benazir Bhutto. To others, the 53-year-old politician is a likeable, experienced and sharp-witted middle-aged man who finally has his chance. For a few, he is all these things at once. One thing is sure: Zardari, who is expected to be elected Pakistan's new president tomorrow, is a highly controversial figure and seems unlikely to be the man who can unite 175 million fractious Pakistanis.

"His first approach is always to make friends - but you wonder when the fist might come out," said Najam Sethi, editor of several newspapers. "He has a kind of natural intelligence and is very charming," said one neutral observer who has known Zardari since childhood. "But it is difficult to know whether his charm is a highly effective act or quite how bright he actually is. He could just be doing a good impression of both."

Such contradictions seem an integral part of the man. Zardari grew up in Karachi, Pakistan's southern port city, son of a landlord and unsuccessful cinema owner. As a teenager, he was known primarily for his skill at polo and driving fast, though his disregard for risk sometimes had a positive side. When riding one evening in the mid-1980s with an elite equestrian club on the outskirts of Karachi, a foreign diplomat's daughter rode into a dangerous swamp. "About 50 people stood around watching as she sank into quicksand," a witness told the Guardian. "Zardari waded in and hauled her out. He's brave."

His arranged marriage in 1987 to Benazir Bhutto, the daughter of former Pakistani prime minister Zulfiqar Ali Bhutto, surprised many and brought him into the central current of his nation's turbulent political life. A year later Bhutto led the Pakistan People's Party (PPP) to victory in elections after the military dictator Zia ul-Haq died in a mysterious plane crash. She promised "food, shelter and clothes" for the masses. Two years later, her administration was dismissed amid allegations of corruption and incompetence. Zardari was imprisoned on charges of blackmail.

Over the next 14 years, Zardari alternated periods of liberty and incarceration. When the PPP regained power in 1993, Bhutto made her husband minister of investment, a controversial decision given the reputation for graft Zardari had acquired during the party's first stint in office. Through the early 1990s he collected a breathtaking range of allegations of corruption, implication in murder, attempted embezzlement and even drug trafficking.

After a second, lengthier stint in prison following Bhutto's second fall from power, Zardari was released after lengthy negotiations between the PPP and General Pervez Musharraf, then in power, in 2004. Zardari dismisses the various charges as political smears and, now that Swiss magistrates who seized millions in accounts linked to Bhutto and her husband have dropped their investigations, there is not a single outstanding case against him. "All through his time in prison he showed an incredible resilience," said one journalist who interviewed him repeatedly. "He always seemed sure the tide would turn."

No one doubts that Zardari is a survivor. Released in 2004, he kept a low profile, undergoing medical treatment in America and only getting involved in active politics shortly before his wife's return to Pakistan in October last year. His reappearance at strategy planning sessions in London dismayed many PPP stalwarts. Power has earned him more supporters, however. "He's an older and wiser man. He's got health problems, he watches his diet, he doesn't drink any longer," said a Pakistani political activist. "He and Benazir were both very young when they came to power. They made mistakes."

A former close associate from the early 1990s spoke, with no obvious irony given the frequent charges of egregious nepotism levelled at Zardari, of how Bhutto's widower "always goes out of his way to help out his friends and even those who somehow reach him for help for jobs, financial help etcetera". But others describe an arrogant, uncultivated and often impatient man ruthless with his enemies. Some accuse Zardari of frequent personal abuse of subordinates. An ally who preferred to remain anonymous said: "He is not a man you want to be on the wrong side of."

Twenty years' experience has taught Zardari much about the brutal game of Pakistani politics. When his wife died, he moved swiftly to secure control of the PPP, sidelining his son and skilfully assuming the Bhutto mantle. However, Zardari is far from an intellectual and does not share his late wife's profound interest in geopolitics or economics, nor her education, eloquence or charisma.

"He doesn't read much, it's true," said an ally. "But you don't necessarily need to be a bookworm type to be president of Pakistan." In the Zardari camp there is quiet jubilation. Few predicted this victory. Zardari will savour it.

CV

Born July 26 1955 in Nawabshah, Pakistan

Family Married Benazir Bhutto in 1987 (she was assassinated in 2007), three children, Bilawal, Bakhtwar and Aseefa

Political career Member national assembly 1990-93 and 1993-96; federal minister of the environment 1993-1996; federal minister of investment 1995-96; transformed the power sector by encouraging investment opportunities. Architect of the Iran-Pakistan gas pipeline. Senator 1997-1999; co-chairman, Pakistan Peoples party from 2008